Are leaf nodes included in the calculation of average branching factor for search trees?
From Wikipedia:


  In computing, tree data structures, and game theory, the branching factor is the number of children at each node, the outdegree. If this value is not uniform, an average branching factor can be calculated.


Outdegree meaning - In case of directed graphs, number of edges going into a node is known as in-degree of the corresponding node and number of edges coming out of a node is known as out-degree of the corresponding node. 

You forgot the outdegree part. In AI we generally draw directed graphs from one state to another, and outdegree is the number of paths leaving a particular node. In your graph direction is not given. Also your graph is not symmetrical, but you can still find out the branching factor (with a little difficulty) of non-symmetrical directed graphs as given here. So technically  your conclusion is correct about leaf nodes not being counted (assuming they are the last state from which no other state can be reached - dead end). Hope this helps!